Two Mississauga sisters were found guilty Thursday of first-degree murder in the 2003 drowning death of their mother.

Prosecutors alleged that the girls plied their mother with drugs and alcohol on Jan. 18, 2003 before one of them held the victim's head underwater until she drowned.

The defence had argued that the mother was an alcoholic who was in a self-induced stupor when she accidentally drowned in the tub.

However, the court also heard that one of the defendants described to a family friend how they had killed the 44-year-old victim and planned to make it look like an accident.

When the friend contacted police, the investigation was reopened into what had been classified as an accidental death.

During the trial, the court saw a videotape in which the defendants separately admitted to the murder.

The defence had argued that the girls falsely claimed to have killed their mother because they craved attention.

The verdict was delivered Thursday in a Brampton court by Justice Bruce Duncan, just a day after closing arguments finished in the case.

"The two defendants set out to commit the perfect crime, but instead they created the perfect prosecution," Duncan said.

"The case against them is overwhelming. It is probably the strongest case I have ever seen in over 30 years of prosecuting, defending and judging criminal cases."

The two suspects, now aged 18 and 19, had pleaded not guilty to the charges.

Neither showed any emotion as the verdict was read.

The two women, who had been under house arrest since their arrests, were taken into custody after the verdict was delivered.

They are expected to be sentenced in the next two or three months.

Because of the sisters' ages at the time of the murder, neither they nor their mother can be identified under the terms of the Youth Criminal Justice Act.

With files from the Canadian Press